Permanent magnets and alloys therefor



Patented Mar. 7, 1950 PERMANENT MAGNETS AND ALLOYS THEREFOR John R. Hansen, Pottersvilie, N. J., assignor to Crucible Steel Company of America, New York, N. Y., a corporation oi New Jersey No Drawing. Application March 16, 1948, Serial No. 15,251

8 Claims.

This invention pertains to improvements in ferrous alloys containing aluminum and nickel adapted for use in permanent magnets and to cast magnets thereof.

More specifically the invention pertains to ferrous alloys and cast magnets of this character containing as essential constituents in addition to iron, about '7 to 20% aluminum and about to 40% nickel, to which may be added any one or more of the following elements within the proportions specified, viz.: up to about 10% each of molybdenum, manganese and vanadium; up to about 5% each of chromium and silicon; up to about 8% tungsten; under 16% cobalt, carbon as low as possible, i. e. about 0.1% maximum, and preferably not more than 0.05%, with the balance substantially iron. For most analyses covered by the invention cobalt will ordinarily be included in amounts ranging from about 0.5% to under 16%. Alloys of the above analyses are ordinarily referred to, as a class, as Alnico alloys, although certain of them contain little or no cobalt.

Alnico alloys, including the above analyses, are characterized as a class in being very brittle and lacking in both hot and cold malleability. They are further not machinable to any practical degree. It is, therefore, the practice to cast permanent magnets thereof essentially to finished shape, break the castings from their respective sprues, and further process by cutting or grinding or both with suitable abrasive wheels. The brittleness is especially manifested by breakage and cracking in casting, grain pickout and edge chipping during grinding and cutting-off operations. This reflects the lack of ductility, coarse grain size, and intergranular weakness prevalent in these alloys. Many service failures involving the use of Alnico permanent magnets are caused by the dislodgement of grains or crystals from the magnet, which then become lodged in the small air gaps employed. This is particularly true in permanent magnet applications, such as in magnetos, where the permanent magnet is the rotor or moving element of the assembly.

Now I have discovered in accordance with one aspect of this invention, that by adding to Alnico alloys of the analyses aforesaid, zirconium in conjunction with titanium, in amounts specified below, that the above mentioned brittleness of the aforementioned Alnico analyses, may be substantially eliminated, with resulting marked reduction in the percentage of rejections of quantity, produced permanent magnets made of such alloys, by reason of such defects as breakage and cracking in castin rain pickout and edge chipeliminate brittleness and rejections based there on, and; in addition, greatly improves the mechanical properties of the alloy in other respects, viz., by imparting hot malleability or workability to the alloy whereby it may be hot formed, and by rendering the alloy machinable.

I have further discovered, curiously enough, that additions of zirconium alone do not improve the mechanical properties of the alloy in the respects above noted, but only when employed in conjunction with titanium. flhe aforesaid improvements in mechanical properties of the alloy, resulting from additions of a. combination of the elements zirconium and titanium, is contrary to expectations in view of the above noted embrittling action of titaniunraione, coupled with the fact that the aforesaid improvements are not achieved by the addition of zirconium alone.

For substantially eliminating brittleness, zirconium and titanium may be added to the above mentioned Alnico alloy in amounts from about .25 to 5% but preferably .25 to 1% of each. For

eliminating grain pickout, best results are secured by adding zirconium and titanium in the proportion of about two parts by weight of zirconium and about three parts by weight of titanium, and within percentage limits of each not exceeding about 1%, as substantiated by the test results hereinafter set forth.

The invention finds particular application in the production of permanent magnets from a cast Alnico alloy containing about 14 to 25% nickel, about 8 to 13% aluminum, about 0.5 to under 16% cobalt, about 2 to 16% copper, and the balance, principally or substantially iron. This alloy, although having excellent magnetic properties, is extremely brittle as cas and hence is subject to rather severe grain plckout during cutting and grinding operations. Additions of titanium and zirconium to this alloy are found not only to substantially eliminate grain pickout in cutting and grinding operations, but in addition render the alloy sufficiently ductile 4 to be susceptible to hot forming operations, and TABLE I also impart sufllcient strength and toughness to the alloy to withstand machining. operations chemmmlym whereby the alloy is rendered machinable, propgreat erties never heretofore obtainable in cast Alnico 5 M Cu 00 M C alloys, insofar as I am aware.

An added advantage resulting from zirconium no .01 0 0 no 5,85 1267 m 015 additions to the Alnico analyses aforesaid, is 33 g 8 1&8? wa l that, as a resulttof stuchtaddistiorifs,t 1the 8:30;; 8g 3 7 g 2 a malizing" or solu ion rea men 0 ese 0 0. .10 .4 10. 0. 1..1) .0 may be carried out at consideratilalig, loweir tetmig g {8:38 3: 13% :33 :83 n e .87 .16 .63 .416 16.90 6.00 12.92 .04 .018 pemtures than otherwise witfhou gr $.58 .08 .03 .243 10.90 0.00 13.00 .05 .010 magnet i i iif'. b t .11.. 1. 14-12 2-2; as -0 ring to the aum num-nc e -co a +001) 10. .r above mentioned, it has heretofore been the prac- 15 13 3:53 :8; :36 :2 2.2g 0 tice to normalize or solution treat this alloy at 14 0.10 .23 .05 .41 17.0 5.20 13.00 .00 .03 temperatures of about 2150 to 2200 F. However, 15 .60 12-99 upon the addition of zirconium in accordance TABLE II Cut-017 and grinding tests Rotary Magnets Heat in )4 Square N Modification 00nd on Cubofl Gut/e Grind Periphery ing Grinding Very Poor Very Poor Good. do do 0 Zr Its 'r1. .74 Zr .32 Ti...

... .do .38 Zr .61 Ti... .do .28 Zr .61 Ti... ....do .44 Zr .60 Tl...

Very Poor.

As Cast Heat Treated. As Cast...-

with the present invention, the permissible normalizing or solution treating temperature is lowered to the range of about 1850 to 1650 F., depending on the particular analysis, as will be shown by the test results presented below.

Reference will now be had to the following Tables I and II, for purposes of comparing the effects on cutoff and grinding operations, 01! adding zirconium and titanium in accordance with the present invention to the aforesaid Alnico analyses, as compared to omitting both of these additions, and also as compared to adding titanium alone. Table I gives the chemical analyses of the various heats tested; while Table II gives the results of cutoff and grinding operations for these heats when cast into magnets of bar /2" square) and rotary magneto types. Referring more specifically to Table II, the results of the cutoff tests are given for the square bar magnets, while for the rotary magneto magnets, results for both gate grinding and periphery grinding operations are presented. In this connection, it will be understood that the cutoff operation consists in cutting the A2" square bar stock, into bar magnets of appropriate lengths. Gate grinding is the operation of grinding off the spur or gate adhering to the magnet after being broken from the sprue; while periphery grinding is the operation of periphery grinding the rotary type magnet to the desired arcuate contour.

Magnet patterns cast Heats l11 inc.Cast as rotary magnets for magnetos and /2" sq. bars.

Heats 12-13 inc.Cast as rotary magnets for magnetos.

Referring to Table II, it will be noted that heat 1, containing no zirconium or titanium, gave very poor results for cutoff and gate grinding operations; while heats 2 to 4 inc., containing no zirconium but progressively increasing amounts of titanium, gave very poor results as regards periphery grinding operations, and only poor or mediocre results on cutoff and gate grinding operations. It should be added that all of heats 1 to 4 inc. showed excessive grain pickout, particularly as regards the titanium-containing analyses, the latter attributable to the presence of insoluble titanium compounds at the grain boundaries. The periphery grinding tests, which reflect the ability of the material to resist grain pickout, were conducted in accordance with acceptance test requirements of magneto manufacturers. The apparent better performance of the titanium-containing analyses, as regards cutoff and gate grinding operations, as compared to periphery grinding, is due both to diflference in standards of acceptance in the two instances, and also to the nature of the operations involved.

Heats 5 and 6, which contained about 0.7% zirconium along with about 0.3% titanium, were entirely free from grain pickout on periphery grinding, and were thus designated as excel- 5 lent," but showed slight heat checking on gate grinding and cutoi! operations. Nevertheless, these heats were-decidedly superior toheats 1 to 4 containing no zirconium.

Uniformly excellent results were obtained tor heats 7, 8 and 10 to 13 inc., containing approximately 0.6% titanium and approximately 0.4% zirconium, a ratio of about three parts titanium to about two parts or zirconium. Heat 9, with only 0.24% zirconium, gave slightly inferior results as regards periphery grinding, although the results were excellent forv cutoff and gate grinding.

Table III below gives the magnetic properties of the various zirconium and titanium containing heats of Tables I and II. Table III shows that these additions in no wise impair the magnetic properties of these Alnico alloys. Furthermore, as shown at the bottom of Table III, the

combined zirconium andtitanium additions, in

accordance with the invention, permit of solution treating or normalizing the alloys at temperatures of about 1650 to 1800 F., as compared to normalizing or solution treating at temperatures of about 2150 to 2200" F. required where the zirconium is omitted.

f Masses per 5.25 to 6.25%; balance iron, except for titanium and with and without zirconium added, as indicated in this table. Also, as indicated in. the table, the rejections are for brittleness, which includes edge chipping and grain pickout. The bar stock, from which these magnets were made, had previously been subjected to centerless grinding and cutofl operations.

TABLE IV Final inspection report--Reiects for brittleness which includes edge chipping and grain pickout on 7;" round a: 1.70- maanets No. 01 Percent e Modification, per cent Magnets Re sets or Inspected B ttienees 0 Zr, .55 Ti 5, 513 9. 0.40 Zr, .55 TI 3, 598 0. 25

duction in rejects in the ratio of about 36:1; or

TABLE III Magnetic properties ggy Modification Condition 13, H. (BH).. X' B.

5 .74 Zr .32 Ti.-- As Cast 6, 450 501 1. 37 3, 850 0.-- 676 Zr .35 Ti do 7,010 608 6--- ---do--.-. Heat treated 7, 250 583 l. 57 4, 500 8.-- 416 Zr .63 As Cast 0, 190 050 8-.. '.do Heat Treated 7,160 559 7 .407 Zr .61 TL--- As Cast 633 1. 49 7.. .do o e o, 1.04 4,350 9.. .243 Zr .63 TL--- 010 ii:- ii: iii in o 1s .44 z: .60 Ti ms 1. 04 5:100 13 .44 Zr .60 Ti L--. 583 1.03 4, 850

1 Normalized l800 5 15" cool to 1100 F.; draw l075-45. 1 Normalized 1800 a0, a cool to 1100 F.; draw l07545'. Normalized 1800 30, 4' 15" cool in'magnetic field; draw l075 F.-45..

4 Normalized l650 30, 2 50" cool in magnetic field; draw 1075" F.45

Bg-B residual. H.-coercive force. B..B value for (Elihu.

By way of demonstrating the tremendous improvement obtained in the quantity production of cast Alnico permanent magnets, containing titanium and zirconium additions in accordance with the present invention, as compared to the quantity production of identical magnets containing no zirconium, the data of the following Table IV is presented, taken from the final inspection reports of the Inspection Department of the Crucible Steel Company of America in the production of 7 round x 1.70" magnets of the followingana vsis, viz.: aluminum 9.5 to 10.5%;

nickel 16.5 to 17.5%; cobalt 12.5 to 13.5%; cop- TABLE V Grain size and R "0 brittleness test Qt" square bars) iifi" A 1'8 16 verage Modiflcation,1er Cent fi gff i. e. lbs. of Grain Di- Grains per smeter Linea] Inch Inches 0 Zr-0 Ti Slight cracking 20 050 o Zr-0.29 Ti--- Cracked badiy-- 31 .021 o Zr-OAS Ti... Cracked 40 y .025 0 Zr0.58 T Cracked badly.. 55 0l8 0 74 Zr-0.32 Ti No cracking... 37 027 o 67 Zr0.33 Ti --do 37 .im 0 4o zi-osi 11.... .do..... 52 .019 0.41 Zr0.63 Ti. -----do 55 .018 0.24 zs-oss Ti..-

Slight cracking 62 .016

l Examined at diameters.

In Table V, the heats tested are those bearins the corresponding numbers in Table I, which latter gives the analysis of each heat. As shown in Table V. all of heats 1 to 4 inc., containing no zirconium, cracked in the indentation test referred to, the cracking being very severe with the titanium-containing heats. As compared to this, heats 5 to 8 inc., containing about 0.4 to 0.7% zirconium and about 0.3 to 0.6% titanium, showed no cracking whatsoever, while heat 9 showed slight cracking due to the low zirconium addition of only 0.24%.

The improvement in intergranular strength resulting from the zirconium additions in accordance with the invention, of titanium-containing alloys of the analyses aforesaid, is due principally to the alloying of the zirconium with the hard, brittle titanium compounds present at the grain boundaries and in the matrix, thereby rendering these alloys ductile. This imparts the desirable property whereby applied stress is dissipated by plastic flow. It has thus been observed in magnets containing zirconium additions in accordance with the invention, that external shock will tend to deform rather than chip or break sharp corners or edges of the magnets.

As above pointed out, zirconium additions in accordance with the invention also confer the property of hot malleability to the Alnico alloys aforesaid. This is demonstrated by the hot bend tests, presented in Table VI below, which were made at various temperatures ranging from about 1850 to 2300 F., with specimens obtained from heat 15, the analysis of which is given in the table. At temperatures of from 2150 to 2300 F., as shown in Table VI, a 180 bend was obtained. Similar tesismade on a like analysis, but omitting zirconium, failed to provide a bend of better than 10 without rupture.

TABLE VI Hot bend test zirconium-containing Alnico The principal obstacle to the successful machining of Alnico alloys is their inherent tendency to breakage and chipping, which is only slightly alleviated by special annealing treatments. Machining tests conducted on heat 14, Table I, in accordance with the present invention, proved that the presence of zirconium permits of machining without breakage and chipping, even in the more brittle or as cast" condition. The tests by which this was demonstrated comprised lathe cutting of A," round stock, using to feeds; and also drilling tests in drilling diameter holes. Sinteredor cemented carbide cutting tools were employed in each operation. Attempts made to lathe-cut 7 round stock, of the heat 14 analysis, but omitting zirconium, invariably resulted in breakage. It should be pointed out, however, in this connection that the hardness of the alloy was not materially affected by the zirconium addition.

What is claimed is: 1. A ferrous alloy adapted for use in permanent magnets, containing: about 7 to 20% aluminum; about 5 to 40% nickel; from about 0.5 to under 16% cobalt; up to about 10% eachof molybdenum, manganese and vanadium; up to about 8% tungsten; up to about 5% each of chromium and silicon; up to about 16% copper; about 0.25 to 5% each of zirconium and titanium; up to about 0.1% carbon; and the remainder iron.

2. A ferrous alloy adapted for use in permanent magnets, containing: about 7 to 20% aluminum; about 5 to 40% nickel; from about 0.5 to under 16% cobalt; up to about 10% each of molybdenum, manganese and vanadium; up to about 8% tungsten; up to about 5% each of chromium and silicon; up to about 16% copper; about 0.25 to 1 each of zirconium and titanium; up to about 0.1% carbon; and the remainder iron.

3. A ferrous alloy adapted for use in permanent magnets, containing: about 7 to 20% aluminum; about 5 to 40% nickel; from about 0.5 to under 16% cobalt; up to about 16% copper; about 0.25 to 1% each of zirconium and titanium; up to about 0.1% carbon; and the balance iron.

4. A ferrous alloy adapted for use in permanent magnets, containing: about 8 to 13% aluminum; about 14 to 30% nickel; from about 0.5 to under 16% cobalt; and 2 to 16% copper; about 0.25 to 1% each of titanium and zirconium; up to about 0.1% carbon; and the bal ance substantially all iron.

5. A permanent magnet made of a ferrous alloy containing: about 7 to 20% aluminum; about 5 to 40% nickel; from about 0.5 to under 16% cobalt; up to about 10% each of molybdenum, manganese and vanadium; -up to about 8% tungsten; up to about 5% each of chromium and silicon; up to about 16% copper; about 0.25 to 5% each of zirconium and titanium; up to about 0.1% carbon; and the remainder iron.

6. A permanent magnet made of a ferrous alloy containing: about 7 to 20% aluminum; about 5 to 40% nickel; from about 0.5 to under 16% cobalt; up to about 10% each of molybdenum, manganese and vanadium; up to about 8% tungsten; up to about 5% each of chromium and silicon; up to about 16% copper; about 0.25 to 1% each of zirconium and titanium; up to about 0.1% carbon; and the remainder iron.

7. A permanent magnet made of a ferrous al- 10y containing: about 7 to 20% aluminum; about 5 to 40% nickel; from about 0.5 to under 16% cobalt; up to about 16% copper; about 0.25 to 1% each of zirconium and titanium; up to about 0.1% carbon; and the balance iron.

' 8. A permanent magnet made of a ferrous alloy containing: about 8 to 13% aluminum; about 14 to 25% nickel; from about 0.5 to under 16% cobalt; about 2 to 16% copper; about 0.25 to 1% each of titanium and zirconium; up to about 0.1% carbon; and the balance substantilaly all iron. JOHN R. HANSEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,027,997 Mishima Jan. 14, 1936 2,027,998 Mishima Jan. 14, 1936 2,156,019 Jonas Apr. 25, 1939 2,185,464 Howell Jan. 2, 1940 2.285.406 Bieber June 9. 1942 Certificate of Correction Patent No. 2,499,862 March 7, 1950 JOHN R. HANSEN It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 6, Table V, in the heading to the fourth column thereof, for lbs. of

read No. of;

and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 18th day of July, A. D. 1950.

[SEAL] JOE E. DANIELS,

Assistant COm'mz'ssioner of Patents. 

1. A FERROUS ALLOY ADAPTED FOR USE INPERMANENT MAGNETS, CONTAINING: ABOUT 7 TO 20% ALUMINUM; ABOUT 5 TO 40% NICKEL; FROM ABOUT 0.5 TO UNDER 16% COBALT; UP TO ABOUT 10% EACH OF MOLYBDENUM, MANGANESE AND VANADIUM; UP TO ABOUT 8% TUNGSTEN; UP TO ABOUT 5% EACH OF CHROMIUM AND SILICON; UP TO ABOUT 16% COPPER; ABOUT 0.25 TO 5% EACH OF ZIRCONIUM AND TITANIUM; UP TO ABOUT 0.1% CARBON; AND THE REMAINDER IRON. 